Silicon-on-Insulator (SOI) technology employs a layer of semiconductor material formed over an insulating layer on a supporting bulk wafer. The structure can be formed by different well-known techniques in the art, for example, separation by implanted oxygen (SIMOX), bonding and etching back (BESOI), and zone melting and recrystallization (ZMR), among others. Typically, the structure comprises a film of monocrystalline silicon formed on a buried layer of silicon dioxide which is formed on a monocrystalline silicon substrate.
One technique for the formation of a SOI substrate by a conventional bonding and etching back method of the prior art is illustrated in FIGS. 1-4. The process starts with the preparation of a silicon substrate 10 (FIG. 1). The silicon substrate 10 is thermally oxidized to grow a layer of silicon oxide 12 (FIG. 1), with a thickness of about 1,000 Å to about 2 μm. Subsequently, a single crystalline silicon substrate 14 is opposed to the silicon oxide layer 12, as shown in FIG. 2. The silicon substrate 10, with the oxide layer 12, is then contacted with the crystalline silicon substrate 14, and the resultant structure is heated to a temperature of about 1000° C., so that the crystalline silicon of the crystalline silicon substrate 14 adheres to the silicon oxide layer 12, as shown in FIG. 3. Next, as illustrated in FIG. 4, the crystalline silicon substrate 14 is polished and its thickness is decreased to approximately 1.5 microns. Thus, a SOI substrate 15 (FIG. 4) is formed of the silicon substrate 10, the silicon oxide layer 12, and the crystalline silicon substrate 14.
Field effect transistors such as MOSFETs, which are fabricated in the upper silicon layer of a SOI structure, such as the SOI substrate 15 of FIG. 4, are known in the art. Typically, MOSFETs are fabricated by placing an undoped polycrystalline material, for example polysilicon, over a relatively thin gate oxide, and implanting the polycrystalline material and adjacent active regions with an impurity dopant material to form source and drain regions. If the impurity dopant material for forming the source/drain regions is n-type, then the resulting MOSFET is an NMOSFET (“NMOS”) device. Conversely, if the impurity dopant material for forming the source/drain regions is p-type, then the resulting MOSFET is a PMOSFET (“PMOS”) device.
Field effect transistors fabricated in the upper silicon layer of a SOI structure, such as the SOI substrate 15 of FIG. 4, have multiple advantages over the transistors fabricated on the conventional bulk silicon substrates. These advantages include, among others, resistance to short-channel effect, increased current drive, higher packing density, and reduced parasitic capacitance. However, despite all these attractive properties, SOI technology still has some drawbacks, which reduce the benefits of using it for high-performance and high-density ultra large scale integrated (ULSI) circuits.
One drawback of the SOI technology is the conductivity of the top silicon layer and its inherent floating body effect, which has particular significance for partially-depleted (PD) or non-fully depleted SOI devices. The floating body effect in such devices occurs as a result of the buried oxide that isolates the channel region of such device and allows charge carriers to build up in the channel region. In a partially-depleted MOSFET, charge carriers (holes in an nMOSFET and electrons in a pMOSFET), generated by impact ionization and drain junction leakage near the drain/body region, accumulate near the source/body region of the transistor. When sufficient carriers accumulate, they are stored in the floating body, which is formed right below the channel region, and alter the floating body potential. As a result, kinks in the I/V curve occur, the threshold voltage is lowered, the dynamic data retention time is altered, and the overall electrical performance of the device may be severely degraded.
One technique for diminishing the negative effects of the charge build up has been to provide an extra electrical connection by adding a contact to the body for hole current collection. However, the currently available hole collection schemes, such as the use of a side-contact, are inefficient, require very complex processing steps, and consume a great amount of device area.
Another technique for diminishing the negative effects of the charge build up has been to use a fully depleted (FD) SOI MOSFET. For this, the silicon layer or island must be sufficiently thin, less than about 400 Angstroms for state-of-the art technology, so that the entire thickness of the body region is depleted of majority carriers and both junctions are at ground. Unfortunately, silicon islands of less than 400 Angstroms thick are extremely costly and difficult to manufacture, due primarily to the sensitivity to variations of silicon film thickness and doping profile across the wafer, as well as to the large source/drain parasitic resistance. In addition, the low threshold voltage (VT) of a conventional fully depleted (FD) SOI causes large subthreshold leakage and low subthreshold voltage (Vt), which in turn greatly reduce the static retention time of a DRAM.
Accordingly, there is a need for an improved method for forming a fully-depleted SOI device having reduced charge build up and accompanying threshold voltage changes and charge leakage. There is also a need for an integrated memory process for SOI wafer fabrication in which access transistors are created with fewer processing steps and which saves wafer area. A defect-free fully-depleted SOI device which is highly immune to soft errors (due to alpha particles and cosmic ray irradiation) is also needed.